The present invention relates to a method and an apparatus for continuously casting metals while preventing the occurrence of various undesired abnormal conditions in the mold surface.
Continuous casting is a method of drawing a molten metal being poured into a casting of constant cross-section, and allows to produce, for example, bars having a circular or rectangular cross-section, pipes and plate-like products. Such a casting method is used for producing castings of, for example, aluminum, copper alloys, cast iron, and steel. To describe continuous casting of steel, for example, a molten material (or melt) is poured from a ladle into a tundish and then from the tundish into a water-cooled mold. A casting emerging out of the mold is supported by a multiplicity of rolls when it is cooled by water. Pinch rolls are disposed under the supporting rolls for slightly pressing the casting to draw it into desired products. The drawn part is cut by a cutter upon reaching a certain length. On the other hand, a lubricant is supplied to inner surfaces of the mold to prevent the casting from sticking thereto. The tundish includes a nozzle to prevent entrainment of impurities into the melt when it enters from the tundish into the mold, so that the melt should be poured with the distal end portion of the nozzle kept always immersed in the melt within the mold.
For example, continuous casting of steel, in particular, has various difficult problems occur which have impeded complete automation of work, especially in the process related to pouring of a melt into a mold. Such problems will be explained in more detail below.
A. Problem of Melt Surface Abnormalities
The associated system including a continuous casting mold (hereinafter referred to simply as mold) comprises 1) a mold, 2) a pouring nozzle, including a lower nozzle, an upper nozzle and a dipped nozzle, which is disposed at the center area of the mold and mounted to the bottom of a tundish, 3) a sliding nozzle (hereinafter referred to simply as SN), or melt (molten steel) flow rate controller utilizing, for example, a stopper, 4) a flow rate controller of gas blown into the pouring nozzle (hereinafter referred to simply as blown gas) for capturing and surfacing inclusions or deoxidation products present in the melt within the mold, and preventing the pouring nozzle from being clogged by the inclusions or deoxidation products, 5) a melt surface level controller, and the like. The blown gas is discharged into the melt surface, which means the surface of contents including molten steel, molten flux and non-molten flux, within the mold, while mold powder or mold flux is charged (or scattered) into the mold for the purposes of heat-keeping, thermal insulation and anti-oxidation of the melt within the mold, capture of deoxidation products or inclusions, and lubrication between a solidified shell and the mold, for instance. The powder or flux is melted upon contacting the melt (or molten steel) to form a molten layer, followed by flowing into gaps between the mold and the solidified shell to effect the various above explained functions. Further, vertical oscillations are applied to the mold to ensure smooth drawing of a casting, while the melt surface within the mold is constantly fluctuated or moved warily with the melt discharged through the pouring nozzle. The melt discharged through the pouring nozzle under such conditions is cooled by the mold and others to start solidifying while forming a meniscus at the top surface of the melt in conformity with the mold.
Thus, in the associated apparatus components and mechanism including a mold, the above mentioned factors are present in a intermixed and complicated state, which factors are delicately balanced in a stable condition of the entire casting system. In other words, the associated apparatus components and mechanisms including a mold contain very complex fluctuating factors, and are always in such an unstable slate that the process is varied largely just by changing some operated variable to a small extent. For example, boiling may occur just by slightly changing a flow rate of the gas blown into the pouring nozzle. The casting system is therefore very sensitive to operational fluctuating factors such as fluctuations in an amount of the melt in the tundish or casting speed, and clogging of the pouring nozzle. Once the balance is lost, there may immediately occur abnormalities, such as level fluctuations, boiling, biased flow and lack of powder, on the melt surface within the mold, such as, so called in this specification melt surface abnormalities. The boiling is a phenomenon that inert gas fed into the tundish, the upper nozzle or the dipped nozzle and then blown into the melt within the mold for the purposes of, e.g., preventing the dipped nozzle from being clogged, is boosted to such an extent as to hinder the melt from flowing from the tundish into the dipped nozzle, and the boosted gas is then blown out at a burst from discharge holes (for example, two) of the dipped nozzle. In the event of this phenomenon, the flames are flared up temporarily from the melt surface and the melt level is lowered. These melt surface abnormalities may directly cause problems including a break-out (hereinafter referred to simply as BO) which is most desirable in the continuous casting operation, and may also directly lead to defect of the surface quality attributable to capture of the powder onto the solidified shell. Thus, keeping the associated apparatus components and mechanisms including a mold stable at all times is the most important point not only in preventing the occurrence of BO, but also in carrying out continuous casting operation steadily while ensuring the quality of castings at the surface and thereabout.
Conventionally, therefore, the melt surface conditions are being monitored within the mold and many detecting means have also been proposed for monitoring. For example, Japanese Patent Unexamined Publication No. 60-49846 discloses a method in which an infrared camera for scanning the melt surface is installed above a mold to measure a temperature distribution over the melt surface thereby to detect a thickness of and its distribution over a powder layer. Japanese Patent Unexamined Publication No. 54-71723 discloses a method of selecting two or more wavelength bands of light radiated from the melt surface within a mold and measuring a temperature of the melt surface from the ratio of light energy level between those wavelength bands, thereby to detect a condition of lack of powder. Japanese Patent Unexamined Publication No. 59-229267 discloses a method of arranging the tip end of an optical fiber to sense (or scan) the melt surface and measuring an electric signal in accordance with an amount of light emitted from the melt surface, thereby to detect a condition of lack of powder. However, those conventional methods do not allow precise detecting of the lack of the powder due to the fact that flames or the like caused upon the gas blown into a pouring nozzle flowing into a mold may erroneously be detected as lack of the powder, or because the melt surface is constantly moved wavily with, for example, oscillations of a mold, the detecting means may be affected by large fluctuations of a temperature condition at the melt surface, or light disturbance caused upon the molten part of the powder or by flames moving warily to appear or disappear with time. Another problem is in that since the temperature measuring device, the infrared camera or the optical fiber, requires a scanning time and a relatively long time for detecting a scattered condition of the powder throughout the melt surface within a mold, the melt surface condition may be changed during the detection process, making it impossible to take a necessary action with the proper timing. Furthermore, in the event of abrupt melt surface abnormalities such as biased flow and boiling mentioned above, the conventional methods can only perform the process of detecting lack of the powder and hence could not detect such abrupt melt surface abnormalities.
Although there have also been proposed techniques of providing thermocouples embedded in a mold wall or mounting a magnetic sensor, an infrared camera or the like above a mold, thereby to a melt surface level, these detecting devices are intended to detect only a melt surface level for controlling constant level, and hence cannot directly detect the melt surface abnormalities such as biased flow, boiling and lack of powder.
As described above, any of the conventional detecting methods had only a single function, and was difficult to quickly and precisely detect a condition of the melt surface in a stable manner. Thus, in actual circumstances various units and devices are arranged intricately in a narrow space near a mold and under high-temperature, dust-full environments, and there is no practical detecting device capable of detecting a condition of the melt surface with high reliability under such unfavorable conditions. To date, therefore, it has been customary for an expert skilled operator to monitor a condition of the melt surface and judge the occurrence of melt surface abnormalities based on his past experience and perception.
As mentioned above, if the associated apparatus components and mechanisms including a mold are out of balance to cause melt surface fluctuations or the melt surface abnormalities such as biased flow, boiling and lack of powder, it has been usual in the past that the operator monitoring the melt surface visually detects such an abnormal condition of the melt surface and immediately takes a proper action based on the detected result. Because the action to balance and stabilize the associated apparatus components and mechanisms including a mold requires quickness and precision, the conventional methods have had to rely on an expert skilled operator, and this has been a big obstacle in an attempt of saving labor expenses.
Notwithstanding such relying on expert skilled operators, there exists a large difference between the operators, and also frequently occurs a delay in detection or erroneous detection, which may cause variations in the quality of castings. In an extreme case, a delay in the proper action may lead to BO.
B. Problems Related to Blow of Inert Gas
In continuous casting of steel, it is common to once store a melt (molten steel) fed through a ladle in a tundish and then pour the melt from the tundish into a mold through a pouring nozzle.
In this case, the melt includes impurities such as deoxidation products, for example, Al.sub.2 O.sub.3, or powder, slag and sulfides (hereinafter referred to collectively to "inclusions"). If any inclusions are captured and left in a casting, this would give rise to various drawbacks for example, causing internal defects called slag intrusion or surface flaws. Further, Al.sub.2 O.sub.3 and the like among the inclusions tend to adhere on the inner surface of the nozzle while passing therethrough and eventually accumulate to such an extent as to clogging the pouring nozzle, whereby the stable operation is hindered n many cases.
It has been known to provide means for effectively separating inclusions from the melt and moving them to the melt surface. For example, Japanese Patent Examined Publication No. 49-28569 discloses a technique of blowing inert gas such as argon or nitrogen gas into a flow of the melt while being poured into a mold, thereby effectively move inclusions to the melt surface. This technique has been adopted widely in recent years. Also, Japanese Utility Model Unexamined Publication No. 62-142463 discloses a device for calculating an appropriate data to control a flow rate of gas based on, for example, the flow rate of the melt determined from the head size of the melt, the width and thickness of castings, and the casting speed.
However, in either prior art where the flow rate of blown gas is visually adjusted by an operator, or where it is automatically controlled using the control device, it has been usual to measure a flow rate of the gas flowing through a pipe for being blown into the melt, to control a value of the gas flow rate. However, the reading on a flow gauge installed in the pipe does not always correspond to the flow rate of the gas actually flowing into the melt, because a portion of the gas may leak during the process before reaching the melt, for example, at refractories employed to form a flow passage, or the pressure loss in the flow passage may be changed. It may also happen that the gas blown into the melt flows into a mold along the wall surface of the pouring nozzle and then escapes above the tundish in vain without effecting the specific function. The ratio of an amount of the gas having been blown into the melt but leaked to an amount of the effective gas having been blown into the melt and reached the mold through the pouring nozzle changes dependent on operating conditions. Therefore, in the conventional techniques it has been very difficult to properly control an amount of the effective gas. If the amount of the effective gas is not properly controlled and the amount of the blown gas exceeds a required level, the flow rate of the gas would become unstable and the melt surface within the mold would be largely disturbed, thereby eventually causing a phenomenon in which the melt would not flow into the pouring nozzle, for example boiling. On the contrary, if the amount of the blown gas is reduced to be too small, the stable operation becomes hard to continue because of clogging of the pouring nozzle and other troubles.
Thus, using the conventional techniques may cause boiling and clogging of the pouring nozzle, due to a difference between an amount of the gas flowing through the pipe and an mount of the effective gas as resulted from measuring a flow rate of the gas in the blowing pipe, or due to difficulties in quantitatively adjusting a flow rate of the gas in a stable manner as experienced even when the operator monitors a condition of the melt surface within the mold and adjusts the flow rate of the gas based on the monitored result.
C. Problem of Slag Beard Formation
As well known, powder is supplied (or scattered) over the melt surface within a mold in continuous casting for the purposes of heat-keeping and air shutdown of the melt, capture of nonmetallic inclusions, and lubrication between a solidified shell and the mold, for instance. The powder is melted upon being subjected to heat of the melt, and fluidized to move from the melt surface along the wall surface of the mold. While moving along the wall surface of the mold, the molten powder is cooled by the mold, but heated by the melt. Meanwhile, vertical oscillations are applied to the mold a solidified shell from sticking to the mold surface. Therefore, the powder, once melted, is caused to become solid again and adhere onto the wall surface of the mold in a region just above the solidifying interface of the melt. Such adhesion of the powder gradually increases an amount of the deposited powder with progress of casting, so that the raw powder projects out of the wall surface of the mold in the form of a terrace, thereby forming a slag beard.
A slag beard is responsible for not only a detrimental effect on the quality of castings, but also more serious trouble in the continuous casting operations. For example, if the melt surface is abruptly raised up for some reason in the presence of a slag beard, the slag beard would be captured by a solidified shell and cause a serious defect in the surface quality of castings. In the worst case, the slag beard captured by the solidified shell just below the mold is forced to melt again upon being subjected to heat of the melt or the solidified shell, which may result in a BO (or break-out), which is more detrimental in the continuous casting operation.
In view of the above, it is most preferable to carry out the operation in such a manner as to avoid the occurrence of a slag beard. However, because of complicated thermal conditions produced just above the solidifying interface, it is very difficult to suppress the occurrence of a slag beard itself.
In the past, however, there have been no methods of detecting a condition of the slag beard formation with good precision. At most, only a method of measuring a temperature of the mold, estimating adhesion of the powder onto the wall surface of the mold based on temperature changes, and predicting formation of a slag beard has been proposed in some cases. Thus, in practice the detecting method using such indirect measuring means is poor in its precision and cannot be adopted in actual apparatus. For the reason, with a technique having been generally adopted to date, an operator directly visually monitors a condition in the mold to detect a condition of the slag beard formation. If a slag beard is detected being formed, the operator pushes or pokes the slag beard by a proper stick to peel it and remove from the wall surface of the mold. This imposes very great physical and mental burden on the operator, and also gives rise to a safety problem. In addition, it has frequently happened that the operator erroneously damages a solidified shell and hence causes a defect in the surface quality of castings. In an extreme case, a BO may be caused. Avoiding such problems requires expert skilled operators, which has been a big obstacle in an attempt of achieving the automated casting operation in continuous casting.
As one of means for removing a slag beard, for example, Japanese Patent Unexamined Publication No. 61-144249 proposes a method of irradiating heat flux such as an infrared ray or laser beam to the slag beard for melting and removing it. However, even such a method has to rely on human efforts to detect the formation of a slag beard, and also requires a great deal of additional energy to melt the slag beard. The case of using a laser beam results in a significant increase in the equipment cost.
D. Problem of Deckel Formation
A Deckel (German) is in the form of so-called "leather cover" which results from solidification of a surface layer of the molten steel or steel bath, when actual and latent heat of the steel bath are removed from the bath surface in a mold for continuous casting. If Deckels are formed, nonmetallic inclusions present in the bath, which should be properly captured by mold powder, would instead be captured by the Deckels to remain in a casting, thereby detrimentally affecting quality of the casting. Also, if a Deckel is formed all over the surface of the steel bath, this would cause more serious operational problems such as a BO and breakage of a dipped nozzle. For the reason, it is required to prevent the formation of a Deckel during continuous casting. However, when the molten steel poured into the mold has a temperature as low as that of the liquid line, or when the casting speed is low, Deckels may be formed because of the reduced temperature of the bath within the mold.
Meanwhile, mold powder is supplied (or scattered) over the bath surface within the mold for the purposes of heat-keeping and thermal insulation of the bath, capture of non-metallic inclusions, and lubrication between a solidified shell and the mold, for instance. Therefore, a condition of the Deckels formation cannot be judged visually. Under such environmental situations, there have been neither devices for detecting the formation of Deckels nor devices for removing the formed Deckels in the past. Thus, it has conventionally been customary for an operator to thrust a proper stick of steel into the melt within the mold for sensing the formation of Deckels based on a feeling perceived by his hands. If Deckels are judged to be formed, the operator pushes the Deckels into the bath using that stick so that it is melted once again.
The foregoing conventional method in which the operator detects and remelts Deckels is inefficient because of manual operation, and the operator is subjected directly to the melt or bath and hence safety is not ensured. In addition, unskilled operators may disturb the solidifying interface between the mold and the solidified shell, thereby causing surface flaws on castings. To improve reliability, therefore, the operation have had to rely on a few skilled operators.
On the other hand, although technical development of achieving the automated casting operation in continuous casting has been actively performed in these years, the operation of detecting the formation of Deckels cannot be automated by utilizing optical detection with image processing, for example, because it is impossible to visually detect the Deckels as mentioned above, and hence has been a big obstacle in an attempt of realizing automated casting operation.
E. Problems Related to Automation of Operations
It can be considered to carry out by a robot operations associated with pouring of a molten steel into a mold. To this end, the above described problems A-D have to be solved. Solving of those problems requires to monitor and recognize process conditions changing from time to time as mentioned in A-D, determine necessary actions based on self-judgment dependent on the recognized conditions, and then implement a plurality of selected actions. However performance and operation control of robots have faced problems as follow.
As to the contents of operation and control relating to utilization of industrial robots, the robots from the simplest ones to those capable of repetitively executing a series of operations, as represented by the teaching and playback system, are now within a practicable range and mainly used. In other words, present industrial robots are applied to only such operations as palletizing, painting, feeding, welding and simple assembling which can be executed through simple control of working positions, and hence can be regarded as robots having not perceptual and judgment abilities, but power of memory alone, because they are controlled only by performing reproductive operations of working timings, working contents and position data which have been taught beforehand. Changes of environments are hardly assumed in use of this type robots, and their operations are carried out essentially under the assumption that the objects to be handled are always in certain positions. Furthermore, being basically capable of repetitive operations as mentioned above, industrial use of those robots is restricted to relatively simple works.
For enlarging an application range of industrial robots, therefore, it has been attempted to use various sensors and control operation of the robot based on information detected by the sensors. More specifically, the attempted method is to detect a condition of operating environments around the robot, and control operation of the robot while determining working timings, working contents and working positions of the robot based on the detected environmental condition. Sensors employed in robots include mainly touch, load (force), visual (image) and audible sensors. As to visual sensors among them, for example, some systems have already been developed to a practicable level through combination of a TV camera and an image processing unit. The contents of visual control are carried out by sequentially recognizing an environmental condition by a visual sensor to determine the process of work, and then issuing a command to a control system. Since the information obtained by the visual sensor ms related to only positions and attitudes, the control to be effected can be achieved through just position control. Stated otherwise, even when a visual function is added to a robot, the control system of robot operation can itself be used as it was. With widespread use of visual sensors using CCDs (see page 37), therefore, operation control of robots utilizing a visual sense is going ahead in an application level of other types sensors. As to load (force) and touch sensors, there exists simple load control of converting only a setting value for a position command based on a load or touch detected signal, and then controlling a position in accordance with the setting value. Although even such simple control can sufficiently execute some works, its application range is restricted. In most cases of using load sensors, it is demanded to continuously control the load so that the vector magnitude in the form of load is controlled to De constant or to provide a predetermined pattern. This type of control is more difficult than the simple load control, and hence problematic under the present state of art. As explained above, operation control of robots utilizing sensors ms now practiced mainly by making use of visual sensors in such a manner as to sequentially control positions, etc. based on detected signals in a large part of applications.
Meanwhile, from the standpoint of multi-functions of robots, there has been used an automatic tool changer (hereinafter referred to as ATC) which has a function capable of easily attaching and detaching tools to the tip end of the robot. But, such multifunctional robots are comparable in level of operation and its control to that as mentioned above, and basically applied to simple works as represented by the teaching and playback system. Operation control of multi-functional robots using sensors (mainly visual sensors) to perform a plurality of works is also comparable in level to that as mentioned above, and limited in application to the field of sequential control.
In short, the technology of utilizing robots remains at the level as mentioned above in the present stage of art. Notwithstanding intensive study for next stage robots, only few have reached a satisfactory level. Stated otherwise, changes of operating environments around robots are not taken into account, and much attention is not paid to the process of recognizing a varying condition of environments and determining the working program, for example, based on the recognized condition. Thus, there are mainly in use robots having no perceptual and judgment abilities necessary to program their own operation control depending on changes of operating environments. At the present stage of art, efficiency and reliability of works executed by robots are regarded as more important than perceptual and judgment abilities thereof.
From the foregoing reasons, conventional robots have faced difficulties in their application to the working process where operating environments to be adopted are changed from time to time, and a plurality of working contents and/or working positions have to be determined by the robots themselves dependent on varying conditions of the operating environments detected by sensors, thereby to perform necessary works. Accordingly, that working process have had to be carried out in such situations through direct operations by plural skilled operators using jigs or the like, or indirect operations by operators using a plurality of manipulators, or introduction of multiple robots as mentioned above for relatively simple works. That working process therefore requires a great deal of equipment and man power costs, and also reduces working efficiency. In addition, a safety problem has been experienced because the operators have had to work in the midst of manipulators and robots.
The present invention has been accomplished in view of the foregoing problems as stated in A-E.